madvise() vs posix_fadvise()

[Dmitry Sivachenko]

On 27 марта 2014 г., at 19:41, John Baldwin <jhb at FreeBSD.org> wrote:
>> 
>> I know about mlock(2), it is a bit overkill.
>> Can someone please explain the difference between madvise(MADV_WILLNEED) and 
> posix_fadvise(POSIX_FADV_WILLNEED)?
> 
> Right now FADV_WILLNEED is a nop.  (I have some patches to implement it for
> UFS.)  I can't recall off the top of my head if MADV_WILLNEED is also a nop.
> However, if both are fully implemented they should be similar in terms of
> requesting async read-ahead.  MADV_WILLNEED might also conceivably
> pre-create PTEs while FADV_WILLNEED can be used on a file that isn't
> mapped but is accessed via read(2).
> 


Hello and thanks for your reply.

Right now I am facing the following problem (stable/10):
There is a (home-grown) webserver which mmap's a large amount of data files (total size is a bit below of RAM, say ~90GB of files with 128GB of RAM).
Server writes access.log (several gigabytes per day).

Some of mmaped data files are used frequently, some are used rarely. On startup, server walks through all of these data files so it's content is read from disk.

After some time of running, I see that rarely used data files are purged from RAM (access to them leads to long-running disk reads) in favour of disk cache
(at 0:00, when I rotate and gzip log file I see Inactive memory goes down to the value of log file size).

Is there any way to tell VM system not to push mmap'ed regions out of RAM in favour of disk caches?